Morphology and rheology of poly(methyl methacrylate)-block-poly ...

1250Macromol. Chem. Phys. 201, 1250–1258 (2000)Full Paper: The phase morphology and rheological propertiesof a series of poly(methyl methacrylate)-blockpoly(isooctylacrylate)-block-poly(methyl methacrylate)triblock copolymers (MIM) have been studied. Thesecopolymers have well-defined molecular structures, witha molecular weight (MW) of poly(methyl methacrylate)(PMMA) in the range of 3500–50000 and MW of poly-(isooctyl acrylate) (PIOA) ranging from 100000 to140000. Atomic force microscopy with phase detectionimaging has shown a two-phase morphology for all theMIM copolymers. The typical spherical, cylindrical, andlamellar phase morphologies have been observed dependingon the copolymer composition. MIM consisting ofvery short PMMA end blocks (MW 3500–5000) behaveas thermoplastic elastomers (TPEs), with however anupper-service temperature higher than the traditional polystyrene-block-polyisoprene-block-polystyreneTPEs (KratonD1107). A higher processing temperature is alsonoted, consistent with the higher viscosity of PMMAcompared to PS.Phase detection image for MIM triblocks: (b) cylindrical morphologyfor sample 5Morphology and rheology of poly(methylmethacrylate)-block-poly(isooctyl acrylate)-blockpoly(methylmethacrylate) triblock copolymers, andpotential as thermoplastic elastomersJ. D. Tong 1 , Ph. Leclère 2 , A. Rasmont 2 , J. L. Brédas 2 , R. Lazzaroni 2 , R. Jérôme* 11Center for Education and Research on Macromolecules (CERM), University of Liège, Sart-Tilman B6, 4000 Liège, Belgium2Service de Chimie des Matériaux Nouveaux, Centre de Recherche en Electronique et Photonique Moléculaires, Universitéde Mons-Hainaut, Place du Parc 20, 7000 Mons, Belgium(Received: March 8, 1999; revised: July 2, 1999)IntroductionRecent developments in ligated anionic polymerization 1)of acrylic monomers have led to the controlled synthesisof new block copolymers, such as poly(methyl methacrylate)-block-poly(alkylacrylate)-block-poly(methyl methacrylate)2) (MAM) and poly(alkyl methacrylate)-blockpolybutadiene-block-poly(alkylmethacrylate) 3) triblocks.These new materials have the potential of replacingadvantageously the traditional styrene-diene based thermoplasticelastomers (TPEs), whose the practical use islimited by the poor oxidation resistance of the centralpolydiene block and by the low service temperature(60l708C) dictated by to the glass transition temperature(T g ) of the outer polystyrene (PS) blocks.Interest for fully acrylic triblock copolymers, thus consistingof inner soft polyacrylate block and outer hardpolymethacrylate blocks, has to be found in the widerange of properties that can be made available merely bychanging the alkyl substituent of the ester group. Forinstance, T g can span a large range from –508C for poly-(isooctyl acrylate) (PIOA) up to 1908C for poly(isobornylmethacrylate). Furthermore, immiscibility of poly(methylmethacrylate) (PMMA) and poly(alkyl acrylates) is therule, although some exceptions may be found in the caseof small alkyl groups and low molecular weight 4) . Themuch better resistance of acrylic polymers to UV and oxidationcompared to polydienes is clearly beneficial. Aprevious paper has reported on the controlled synthesisMacromol. Chem. Phys. 201, No. 12 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2000 1022-1352/2000/1208–1250$17.50+.50/0

Morphology and rheology of poly(methyl methacrylate) ... 1251and mechanical properties of PMMA-block-PIOA-block-PMMA triblock copolymers (MIM) 5) . The phase morphologyand the rheological properties of these MIM triblockcopolymers have been analyzed further and are thetopic of this paper.Experimental partMaterialsMIM triblock and MI diblock copolymers were synthesizedby sequential anionic polymerization of MMA, tert-butylacrylate (tBA) (and MMA, in case of triblocks) respectively,followed by the acid-catalyzed transalcoholysis of the tertbutylester groups by isooctyl alcohol. The detailed synthesiswas reported elsewhere 2, 5) . The molecular characteristics ofthe MIM triblocks and the MI diblock are listed in Tab. 1. Acommercial grade polystyrene-block-polyisoprene(PIP)-block-polystyrene triblock copolymer, SIPS (Kraton D1107from Shell Development Company, 15l18 wt.-% uncoupleddiblock), was used for the sake of comparison. Theannounced molecular weight (MW) was 10000–120000–10000, with a polystyrene content of 15 wt.-%.was recorded in the so-called “soft tapping mode” 6) in orderto avoid deformation and indentation of the polymer surfaceby the tip. All the images were recorded with the maximumavailable number of pixels (512) in each direction. Forimage analysis, the Nanoscope image processing softwarewas used. The images were usually reported as captured,repeated scans assessing the reproducibility of the observedstructures.Rheological measurementsThe RSI ARES rheometer from Rheometrics equipped witha force balance transducer was used, either in the cone-platemode: (plate: 25 mm diameter, cone: 48 angle, gap: 56 lmbetween the cone tip and the plate; for samples 1 and 7) or inthe parallel plate mode (25 mm diameter, for all the othersamples). The temperature control was better than 18C. Theapplied strain was always kept within the linear viscoelasticregime, so that the phase morphology did not change undershearing. The frequency range was between0.1 Hzl16.7 Hz. A Polymer Laboratory DMTA (parallelplate with 7 mm diameter) was used to conduct temperaturesweep experiments at 1 Hz (ramp mode, heating rate of 28C/min).Sample preparationThin MIM films were cast on mica from dilute solution inTHF (2 mg/ml) and sheltered from dust throughout. THFwas let to evaporate very slowly for a few days. The filmswere annealed at 1408C under high vacuum for 24 h beforeAFM observation. The film thickness was ca. 500 nm.Longer annealing times and longer thickness did not changethe microphase morphology. Some topological defects werehowever observed for 1–2 mm thick samples. Films suitedto rheological testing, were prepared by casting a copolymersolution (8 wt.-%; 160 ml) in a 100 mm diameter polyethylenedish. The solvent was evaporated over 4 days at roomtemperature. Ca. 1.5 mm films were dried to constant weightin a vacuum oven at 808C for ca. 1 day. The reason formilder conditions compared to the preparation of films forAFM observation must be found in the possible dehydrationat 1408C of the residual carboxylic acids left by the transalcoholysisreaction, which might affect the rheological data incontrast to the already set up phase morphology. Accordingly,rheology will not be discussed in direct relation to themicrophase morphology observed by AFM. The specimenswere colorless, transparent and elastomeric with a smoothsurface.AFM observationAll the AFM images were recorded with a Nanoscope IIIamicroscope from Digital Instruments Inc. operated in theTapping Mode (at 258C, in air). Microfabricated cantileverswere used with a spring constant of 30 N N m –1 . The instrumentis equipped with the Extender TM Electronics Module,such that height and phase cartographies can be simultaneouslyrecorded. Several areas of the same sample wereobserved, with scanning time of ca. 5 min. The phase imageResults and discussionPhase morphologyThe very low electronic contrast between the constitutiveblocks of poly alkyl(meth)acrylate-containing triblocks isquite a problem for the observation of nanophase-separatedmorphology by transmission electron microscopy(TEM) and small angle X-ray scattering (SAXS). Onlyindirect techniques, such as NMR 7) and DMTA 5) , havebeen used for this purpose. However, atomic force microscopy(AFM) with phase detection imaging has provedvery recently to be very appropriate to the direct observationof phase separation in fully acrylic block copolymers8) . In this work, tapping-mode atomic force microscopy(TMAFM) with phase detection imaging (PDI) hasbeen used, this technique having proven high efficiencyfor the analysis of the phase morphology of polystyreneblock-polyisoprene-block-polystyrenecopolymers 9) .Fig. 1 shows typical AFM images for the MIM triblockcontaining 6.5 wt% PMMA (sample 1, Tab. 1). Theheight image (Fig. 1a) is very uniform, thus indicatingthat the sample surface is very smooth, in line with ca.1.4 nm root mean square roughness for a 161 lm 2 area.This preliminary observation is a guarantee that any contrastobserved in the PDI image will not originate fromdifferences in the surface topography. The phase image(Fig. 1b) clearly shows a two-phase morphology for thesample 1, that consists of bright spheres randomly distributedin a dark matrix. Recent models proposed 10) toaccount for the phase contrast in TMAFM indicate that in“soft tapping” operation, the phase shift is directly related

Morphology and rheology of poly(methyl methacrylate) ... 1253The equilibrium phase morphology of block copolymersstrongly depends on the copolymer composition, asillustrated by the styrene-diene block copolymers 11, 12) .Spherical morphology is commonly observed for polystyrene(PS) contents up to l17 wt%. When the PS contentranges from 17 to 38 wt.-%, the phase morphology iscylindrical, whereas a lamellar morphology is reportedfor 36l62 wt.-% PS. For the sake of comparison, a seriesof MIM copolymers covering a wide range of PMMAcomposition has been analyzed by AFM. Fig. 2 showssome typical phase morphologies: PMMA spheres(Fig. 2a) for sample 3 of low PMMA content (12 wt.-%),PMMA cylinders (Fig. 2b) for sample 5 of intermediatePMMA content (22 wt.-%), and PMMA short lamellae(Fig. 2c) for sample 6 of higher PMMA content (36 wt.-%). Although the copolymer composition-phase morphologyrelationship is basically comparable for MIM andstyrene-diene block copolymers, MIM with very shortPMMA blocks (M — n: 3500) shows a well-defined sphericalmorphology, in contrast to the PS-block-polybutadiene(PB)-b-PS (SBS) and SIPS analogues 13) that show nophase separation.The annealing of sample 5 has a strong effect on theorientation of the PMMA cylinders with respect to thesample surface. Before annealing, the cylinders lie flat onthe surface (Fig. 2b). After annealing at 1408C, only afew flat cylinders coexist with many bright dots whosediameter is similar to the width of the flat cylinders. Thisobservation is consistent with the fact that the bright dotsare cylinders standing upright perpendicular to the surface(Fig. 2d). This reorganization is likely to be governedby the surface energy, which is smaller for PIOA(30610 –3 N/m) than for PMMA (41610 –3 N/m). As theequilibrium is approached, the PMMA cylinders tend toreorganize themselves with their apex at the surface.Viscoelastic behavior of MIM triblock copolymersIt is well established that the domain structure of blockcopolymers, such as SBS and SIPS, persists beyond theupper (PS) glass transition temperature. However, as temperatureis raised above a critical value, the microdomainstructure disappears completely, resulting in a homogeneoussystem. This transition is referred to as the orderdisordertransition (ODT), which occurs in all knownblock copolymers 14–18) . The viscoelastic behavior of theseblock copolymers significantly changes at the ODT, theblock copolymer being easily processable beyond ODT.Dynamic temperature sweep experimentsFig. 3 compares the temperature dependence of the storagemodulus (G9) and the loss factor (tand) measured at1 Hz for the 5000–140000–5000 MIM triblock (sample2, Tab. 1) and the Kraton D1107 sample. The dynamicFig. 3. Temperature dependence of the shear storage modulus(G9) and the loss factor (tan d) (1 Hz, heating rate: 28C/m). (a)MIM triblock, sample 2; (b) Kraton D1107mechanical behavior of these samples is typical of thermoplasticelastomers, i.e. two transitions, a rubbery plateaubetween them and the terminal zone at higher temperature.Compared to Kraton D1107, the MIM sample(Fig. 3a) has quite a comparable behavior, except for theG9 plateau region which is more flat (up to 1108C) asresult of the absence of diblock copolymers 19) . The tandcurves do not exhibit any clear peak at the T g of PS (orPMMA) microdomains, which can be explained by thelow PS (or PMMA) content and the simultaneous occurrenceof the flow.Fig. 4 compares the logG9 vs. temperature curves for aseries of MIM triblock copolymers covering a large rangeof PMMA molecular weight (3500–20000). The behaviorof MIM triblocks is similar to that of the Kraton TPEwhen the PMMA molecular weight is smaller than 7000.However, when this molecular weight is higher, no terminalzone is observed up to 2008C, the highest tempera-

1254 J. D. Tong et al.Fig. 4. Temperature dependence of the shear storage modulus(G9) for a series of MIM triblocks at 1 Hz (heating rate: 28C/m).For the sake of clarity, curves have been vertically shifted withrespect to sample 2 (sample 1 downwards by 0.5 unit; samples3, 4 and 5, upwards by 0.5, 1.0 and 1.5 units, respectivelyture tested. It is known that G9 measured at low frequenciesdecreases sharply at or near the order-disorder transitiontemperature (T ODT ) of the block copolymer 20 – 23) . It isthus obvious that a phase-separated morphology persistsbeyond the glass transition of the PMMA microdomainsof samples 3 to 5, at least up to 2008C. The comparisonof MIM and SIPS of comparable molecular structure(sample 4 and Kraton D1107), also shows that G9 starts todecrease only at 1208C for MIM, which is ca. 208Chigher than for the SIPS sample, in agreement with ahigher service temperature for the MIM triblock copolymer.Relaxation at temperatures beyond T g of PMMAThe rheological properties of polymers are closely relatedto the relaxation process of the polymer chains. Recently,Berglund and McKay have thoroughly studied the relaxationbehavior of SIPS triblock copolymers 19) . The completerelaxation stress proceeds in two steps:diffusion ofthe outer blocks out of the microdomains, followed bydiffusion of the released chains through the entangledmidblock chains. Fig. 5 compares the relaxation for twoMIM triblocks of different PMMA molecular weight(samples 1 and 3) but of the same spherical morphology.The relaxation curve for the MIM containing PMMAblocks of 7000 MW clearly confirms Berglund andMcKay’s conclusion, i.e. a two-step drop of G(t) in the10 –1 l10 1 s and 10 1 l2610 2 s regions, respectively,assigned to the relaxation of the PMMA blocks out of theFig. 5. Time dependence of the stress modulus for two MIMtriblocks (samples 1 and 3, in Tab. 1)hard microdomains followed by the diffusion of the entirechains through the PIOA matrix. The relaxation of MIMcontaining two times shorter PMMA blocks is quite reminiscentof that one commonly observed for monophasepolymers, so indicating that no microdomain structurepersists at 1308C.Dynamic frequency sweep experimentsFig. 6 illustrates how the complex viscosity of sample 2depends on the angular frequency. The complex viscosityat 1208C is clearly non-Newtonian, as is the case for vulcanizedrubber. A yield behavior starts to be observed at1808C, and a Newtonian behavior at low shear rate, aswell. The observation that the Newtonian behaviorbecomes more pronounced as the temperature isincreased beyond some limit, is the signature of the completerelaxation of the triblock chains at low angular frequencywhen T ODT of the block copolymer isapproached 19) . Fig. 7 compares the plots of complex viscosityvs. angular frequency for a series of MIM triblocksat 1708C. As the PMMA molecular weight is increased,the non-Newtonian behavior is continuously more pronounced,as a result of increasingly more extended phaseseparation when the molecular weight 24) is increased.Fig. 8 compares the complex viscosity of MIM/MI binaryblend (sample 9, Tab. 1) and Kraton D1107. It mustbe noted that although the MIM/MI binary blend containsthe same diblock content as the Kraton copolymer, thecontent of the hard block (ca. 11.5 wt.-%) is smaller comparedto Kraton (ca. 15 wt.-%). Moreover, the molecularweight between chain entanglements (M e ) is 60000 forPIOA 6) much higher than the 6100 for PIP 2) . For these

Morphology and rheology of poly(methyl methacrylate) ... 1255Fig. 6. Plots of complex viscosity vs. angular frequency for theMIM sample 2Fig. 8. Plots of complex viscosity vs. angular frequency for theMIM/MI binary blend (sample 9, open symbols) and KratonD1107 (filled symbols)Fig. 7. Plots of complex viscosity vs. angular frequency for aseries of MIM triblocks (Tab. 1) at 1808Ctwo reasons, not only the shear modulus but also the complexviscosity are expected to be higher for the Kratoncopolymer than for the MIM/MI counterpart. This expectationis confirmed for the complex viscosity at 1508C athigh shear rates. However, as the temperature isincreased, the complex viscosity of sample 9 remainshigher compared to the Kraton sample in a larger rangeof shear rates, indicating more restricted relaxation andbetter persistence of the microdomain structures for theMIM copolymer at high temperatures.Order-disorder transition temperatureAs long as the microdomains persist beyond T g ofPMMA, they contribute to keeping the melt viscosityFig. 9. logG9 vs. logG99 for the MIM sample 1. The slope is 2at 1708C and 1808Chigh 26) , which is undesirable for the processing of theMIM copolymers. Han and co-workers 27) have recentlyshown that rheological data can be used to determine theorder-disorder transition temperature, as illustrated inFig. 9, where logG9 is plotted vs. logG99 at different temperatures.According to these authors, the threshold temperatureat which the logG9 vs. logG99 plot becomes linearand the slope (of 2) becomes independent of temperatureis the signature of the order-disorder transition. Thisprediction is based on a rheological model suited tohomogeneous polymers in the terminal zone (Eq. (1)):logG9 = 2logG99 – log(qRT/M e ) + log(p 2 /8) (1)

1256 J. D. Tong et al.MIM triblocks. Data for SIPS copolymers are alsoreported from the scientific literature 19, 27, 28) . It is clearthat the T ODT of MIM triblocks is much higher than T ODTof the SIPS analogues, this might be a problem for thecopolymer processing, since the degradation temperatureof the polyacrylate central block is only 2308C as measuredby TGA (58/min, N 2 ).Viscoelastic properties of polystyrene and poly(methylmethacrylate) homopolymersThe origin of the difference in T ODT of MIM and SIPScopolymers might be found in the polymer-polymer interactionparameter, v ab , for the PMMA/PIOA and the PS/PIP pairs. The v ab value is commonly determined fromthe solubility parameter for each component at constantmolecular weight 24) :Fig. 10. logG9 vs. logG99 for the MIM sample 4Tab. 2. T ODT for MIM and SIS block copolymers a)Sample M — n610 –3 Content ofhard block inwt.-%T ODT /8CSIS-1 28) 7.4–99–7.4 11.4 180SIS-2 19) 10.6–125.4–10.6 14.5 280Kraton 10–120–10 + 14.3 230D1107 27) (15l18% diblock)MIM-1 3.5–100–3.5 6.5 130–140MIM-2 5–140–5 6.6 200MIM-3 7–100–7 12.2 A200MIM-4 10–140–10 12.5 A200MIM-9 10–140–10 +(17% diblock)11.5 A200a)Measured by Han’s method 18) .where q is the polymer density, R the gas constant, T theabsolute temperature and M e the molecular weightbetween chain entanglements. Fig. 9 shows that plots oflogG9 vs. logG99 for the MIM sample 1 (Tab. 1) are notlinear (data collected from frequency sweep experiments)below 130–1408C, particularly at higher frequency(upper part of the curves). As the temperature exceeds130–1408C, the logG9 vs. logG99 curve is linear with aslope of 2, so indicating that T ODT lies in this temperaturerange, at least in the range of the investigated frequencies(0.1l15.9 Hz). Fig. 10 illustrates the same relationshipfor the MIM sample 4, which contains more PMMA (12wt.-%) of higher molecular weight (7000) although preservingthe same spherical morphology. Plots of logG9vs. logG99 are far from linearity and superposition, indicatingthat T ODT is well beyond the range of the investigatedtemperatures (150–2008C). Tab. 2 lists the valuesof T ODT , as determined by Han’s method for a series ofv ab = M a (d a –d b ) 2 /q a RTwhere M a the is molecular weight of component a; d a andd b are the solubility parameters of components a and b; q ais the density of component a. The solubility parametersfor PMMA, poly(alkyl acrylates) (e.g. ethyl, propyl,butyl), PS, and PIP are 18.6, 18.0l19.8, 19.0, and16l17 (J/cm 3 ) 1/2, 29) respectively. Thus, d a –d b for thePMMA/PIOA pair is expected to be smaller than for thePS/PIP pair, although the exact value of d for PIOA isunknown. The smaller polymer-polymer interaction parameterfor the PMMA/PIOA pair compared to the PS/PIPpair is thus in favor of a lower degree of immiscibilityand thus a lower order-disorder transition temperature forthe MIM copolymers, which completely disagrees withthe experimental observations.The physico-mechanical characteristics of the hardblocks may not be ignored when the melt processing ofthe triblock TPEs are concerned. Although the volumeand molecular weight of the monomer units and T g arecomparable for PMMA and PS, these two polymers havequite a different behavior in solution and in the melt. Forexample, Tab. 3 shows that M e for PS is roughly threetimes as large as M e for PMMA and that the viscoelasticcoefficients extracted from the WLF equation are alsodifferent 30 – 35) . The viscoelastic characteristics for PMMAand PS (Tab. 3) have been reported for samples preparedby free-radical polymerization, thus for samples of verybroad molecular weight distribution, which might explainthe dispersion of some rheological properties dependingon their origin. In order to improve the accuracy of theM e values, PMMA (M — n: 5000 to 80000) and PS (M — n:10000) homopolymers have been prepared, in this study,by anionic polymerization, so making samples of verynarrow molecular weight distribution (a1.1) available.M e for PMMA has been found to be 6000, consistent withthe previously reported data 30) . Tab. 4 provides the zero-

Morphology and rheology of poly(methyl methacrylate) ... 1257Tab. 3. Viscoelastic characteristics for PMMA and PS a)Sample G 0 N 6105PaM eg=molb†M cg=molc) c†C 1 C 2CPMMA 6.57 4 700l9 200 17 600l27 500 9.0 13.4PS 1.99 17 300l18 700 31 200l32 800 35.5 55a)Data from refs. 30–35)b)Critical molecular weight at which the molecular weight (M)dependence of g 0 changes from g 0 = KM (M a Mc) to g 0 =K9M 3.4 (M A Mc). g 0 is the zero-shear viscosity defined as g 0 =lim xe0 (G99/x), with x the shear rate and G99 the shear lossmodulus.c)Coefficients of the WLF equation, –loga T = C 1 (T–T 0 )/(C 2 +T–T 0 )Tab. 4.Zero-shear viscosity for PMMA and PS at 1708Ca)1808C.b)1838C 36) .Sample M — n and (M — w/M — n) g 0 /(Pa N s)PMMA 5000 (1.10) 1800PMMA 8000 (1.07) 3700PMMA 10000 (1.05) 7000PMMA 20000 (1.04) 80000 a)PS 10000 (1.10) 44PS 48500 (1.10) 1500 b)shear viscosity (g 0 ) measured at 1708C for these PMMAand PS samples. The experimental viscosity for PMMAof 10000 MW is more than 150 times larger compared toPS of the same MW. Even when the PMMA MW is halfthat of the PS, so leading to comparable T g ’s, the g 0 valuefor PMMA is still 40 times as large as for PS. Fig. 11compares the plots of complex viscosity vs. angular frequencyfor PS (Fig. 11a) and PMMA (Fig. 11b) of 10000MW (thus the same MW as the PS block of KratonD1107 and the PMMA block of MIM (sample 4)). ForPMMA to have the same melt viscosity as PS, it must beheated at least 408C higher than PS.The much higher zero-shear viscosity and lower M eindicate that PMMA is less prone to flow than PS, whichmight explain why the MIM triblocks have to be processedat higher temperature than the SIPS analogues,consistent with a higher order-disorder transition temperature.Thus, although the T ODT can be theoretically predicted15) from the polymer-polymer interaction parameterand the block copolymer composition, the occurrence ofthis transition in MIM is much delayed by unfavorablekinetic parameters.ConclusionFig. 11. Plot of melt viscosity vs. angular frequency for: (a)10000 PS; (b) 10000 PMMAThe phase morphology of poly(methyl methacrylate)-block-poly(isooctyl acrylate)-block-poly(methyl methacrylate)triblock copolymers has been studied by atomicforce microscopy with phase detection imaging. Spherical,cylindrical, and lamellar morphologies have beenobserved for block copolymers of increasing PMMA content.These copolymers exhibit a behavior typical of thermoplasticelastomers only when the PMMA molecularweight is small (3500 and 5000). Otherwise, the microdomainstructure of the MIM triblocks persists beyondthe glass transition temperature of PMMA. The muchhigher experimental T ODT for MIMs compared to the SIPSanalogues is not of thermodynamic origin but rather dueto kinetic factors in relation to the high zero-shear viscosityand low M e for the outer PMMA blocks.

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